The present invention relates to polypropylene-based resin compositions for use in extrusion processes. These compositions find use in body side molding applications commonly employed in, for example, the automotive parts industry. These compositions can replace parts typically comprising other thermoplastic materials such as polyvinyl chloride. The body side moldings of the present invention are produced by co-extrusion. The co-extrudate comprises a core layer; and a skin layer that optionally contains metallic pigment. The core and skin materials comprise a thermoplastic olefin ("TPO") compound and polypropylene, respectively.
It has been typical in the automotive parts industry to manufacture exterior parts from various types of polymer materials. These polymer materials have significant advantages over more traditional materials such as metals. For instance, the polymer materials are lighter weight, and are often cheaper to produce. These materials may also have the advantage of being recyclable.
One of the first materials used in automotive exterior parts applications was polyvinyl chloride (PVC). Some of the advantages of PVC include dimensional stability, flexibility, receptiveness to paint and/or other coating materials, and compatibility with foam tapes which are typically used to attach the body side molding to the automobile. PVC has certain drawbacks. These include difficulties in processability, and environmental concerns associated with the material itself. As a result, many attempts have been made to replace PVC exterior automotive parts with parts made from other polymer materials. These polymers have included polypropylene.
Polypropylene resin materials have been designed for bumper covers, bumper fascia, spoilers, body side cladding or molding, and so forth. However, there are certain drawbacks to polypropylene resins which are not inherent to PVC. PVC is flexible and forms a good part when extruded and subsequently quenched from the melt.
Unlike PVC, polypropylene is brittle and distorted due to shrinkage when quenched from the melt. Polypropylene thus requires various other components to be incorporated to reduce brittleness and shrinkage. To reduce brittleness, especially at a low temperature, rubber or C.sub.2 -C.sub.8 olefin copolymers are employed as impact modifiers. However, the presence of impact modifiers in polypropylene decreases the stiffness of the compound. Therefore, inorganic fillers such as talc are incorporated in order to achieve a balance between impact resistance and stiffness. Also, these fillers tend to reduce shrinkage of the final product. In order to minimize distortion, slow cooling, preferably via air, is often employed. However, slow air cooling requires relatively long cooling time before subsequent processing.
It is known in the art that polypropylene resin products can be formed via several distinct processes. These include molding processes. It is known in the art that body side exterior parts comprising polypropylene can be manufactured using various types of molding operations. Polypropylene resins utilized in making molded products are disclosed in the following references U.S. Pat. Nos. 5,744,535, 5,773,515, and 5,763,520. Additionally, EP 0869143, WO 9827154, and WO 9831744 disclose molded products made from polypropylene based resins.
More recently, U.S. Pat. No. 5,800,912 discloses a high gloss molded resin for use as an exterior body side molding for automobiles. The disclosed resins are disclosed as having high gloss, metallic luster, excellent weather-proofing and scratch resistant properties. The product materials are made via an injection molding process. The disclosed product is said to have two layers. The outer layer is referred to as the skin layer. The inner layer is referred to as the core layer. The physical and chemical properties of each layer are different. The skin layer provides a finished product having the desired external properties described above (i.e., high gloss, weather and scratch resistance). The core layer provides the flexibility and processability necessary to make a functional molded product. The polypropylene resins utilized have a minimum melt flow rate (MFR) of 30. This is desirable for the injection molding operations.
One commonality among the above references is the exclusive use of one or more injection molding process. However, injection molding processes are not always the most desirable processing method in the art. Other methods of processing polypropylene resins are known and desirable in the art. These include extrusion processes.
It is known in the art that polypropylene resins can be used to make finished products via extrusion processes. Extrusion processes have certain advantages over molding operations. In particular, extrusion manufacturing is generally less expensive than molding processes. Therefore, side molding parts made via extrusion will have a competitive advantage over similar molded parts.
It is also known in the art that an injection-molding grade polypropylene can not be used for extrusion process. In the context of producing exterior body side moldings, the processing window of the resin material is too narrow. The finished product will not have the desired physical properties mentioned above.
Therefore, it is an object of the present invention to provide a PVC replacement material suitable for manufacturing automotive exterior parts. Preferably, the exterior part is a body side molding.
It is another object of the present invention to provide polypropylene-based resin compositions suitable for use in making these automotive exterior parts.
It is a further object of the present invention to provide polypropylene-based resin compositions suitable for use in extrusion processes.
It is also an objective of the present invention to provide an automotive exterior part comprising two distinct layers.